1. Field of the Invention
The present invention relates to a method of charging and discharging a battery having a plurality of electrochemical cells which dissolve an active material from the electrodes into the electrolyte during the discharging process and deposit the active material on the surfaces of the electrodes during the charging process and an apparatus adopting this method. More particularly, the present invention relates to such a method and apparatus for achieving full discharge in order to equalize the electrodes.
2. Description of the Prior Art
In the apparatus such as an electric vehicle which supplies an electric power to the load by using a battery, it is very important in terms of the performance of the apparatus to enlarge the capacity and prolong the life of the battery.
As a battery which is capable of effectively supplying a large capacity of electric power, a metal-halogen battery such as a zinc-bromine battery having a plurality of electrochemical cells which dissolve an active material from the electrodes into the electrolyte during the discharging process and deposit the active material on the surfaces of the electrodes in the charging process has been developed. A battery of this system is disclosed in, for example, Japanese Patent Laid-Open No. 132775/1981. The prior art will be explained in the following with reference to FIG. 12.
In FIG. 12, the reference numeral 112 represents a container of a electrochemical cell which is a structural unit of a battery. The container 112 accommodates a negative electrode 114, a positive electrode 116 and a separator film 118 for separating the negative electrode 114 and the positive electrode 116 from each other.
To the negative electrode 114, an electrolyte on the negative electrode side is supplied from an electrolyte tank 122 through a pipe 120 and a pump 124, and to the positive electrode 116, an electrolyte on the positive electrode side is supplied from an electrolyte tank 130 through a pipe 128 and a pump 126. The electrolyte tank 130 on the positive electrode side may accommodate a complex tank (not shown) which stores bromine in the form of a complex. The positive electrode 116 and the negative electrode 114 are composed of a conductive resin material such as carbon plastic for the purpose of lightening the weight.
In such a cell, during charging, the Zn.sup.2+ ions in the electrolyte are deposited on the negative electrode 114 as Zn, and Br.sup.- ions are changed into Br.sub.2 on the side of the positive electrode 116 and dispersed in the electrolyte in the form of a complex. On the other hand, during discharging, Zn deposited on the negative electrode 114 dissolves in the electrolyte in the form of Zn.sup.2+ ions, and Br.sub.2 dissolves in the electrolyte in the form of Br.sup.- ions A battery practically used is composed of a multiplicity of such cells laminated in series in order to obtain a desired output voltage.
In this case, it is only the containers 112, the negative electrodes 114, the positive electrodes 116 and the separator films 118 that are laminated as the cells, and the pipes 120 and 128, the electrolyte tanks 122 and 130 and the pumps 124 and 126, etc. are not laminated but used in common.
In the above-described battery, the configurations of the negative electrodes and the positive electrodes and the natures of the separator films slightly differ with cells. For this reason, the natures of the cells are not exactly the same, which disadvantageously causes a difference in amount of self-discharge.
Therefore, in a battery composed of a plurality of such cells connected in series, the difference in the amount of Zn deposit, namely, the difference in the amount of charge increases with the lapse of time whether it is in operation or out of operation, as indicated by the solid line in FIG. 7. As a result, the total capacity and the voltage of the battery are reduced.
It is also known that not only is there a nonuniformity of the amount of Zn deposit between the cells, but also there is a difference in the amount of Zn deposited on the surface of the negative electrode in one cell depending on the locality, which may cause a trouble such as a breakage of the separator film due to the growth of dendrites during repetitions of charge and discharge.
Conventionally, as a measure for eliminating the nonuniformity of the amount of Zn deposit and depositing Zn on the negative electrodes to a uniform thickness (hereinunder referred to as "equalization"), the battery before charge has been fully discharged so as to dissolve the total amount of Zn deposited on the negative electrodes in the electrolyte. According to a method disclosed in Japanese Patent Laid-Open No. 132775/1981, the battery is divided into a plurality of battery blocks and the discharge current is increased for achieving full discharge in any given block in comparison with the other blocks in order to achieve the full discharge of the battery in operation. Japanese Patent Laid-Open No. 49577/1985 discloses a method of achieving the full discharge of a given one battery block in the same structure by feeding from the battery block to an auxiliary battery.
However, when these methods are applied to an electric vehicle, which generally travels a short distance per day, it is often the case that the full discharge is not achieved even in one battery block among all the blocks during operation of the battery, namely, between the completion of the charge of the battery and the initiation of the next charge.
Accordingly, if the full discharge for equalization is carried out in all the blocks of the battery, a large capacity remains in the battery and since the electric power is consumed by the auxiliary equipment, it cannot afford a sufficiently large discharge current, so that a long time is required for realizing the full discharge, which makes it impossible to prevent the growth of dendrites.
If the battery is continuously operated until the battery is sufficiently discharged in order to eliminate this problem, there is much risk of the battery dying during the operation and disabling the electric vehicle from running.
As a usage of the battery, a method of not operating all the blocks equally but fully discharging the blocks one by one may be considered. This method, however, is unsuitable for practical use, because if a part of the blocks of the battery are discharged, the blocks lose the capacity of energy supply for running, thereby greatly reducing the output of the battery and the driving performance thereafter.
Especially, in the case of an electric vehicle, since the capacity of the battery mounted thereon is limited, it is desirable that the maximum electric power is constantly supplied from all the blocks of the battery.